In conventional radiography, films containing light-sensitive silver-halide grains are employed in a number of image recording devices including but not limited to x-ray imaging cassettes, film based dosimeters and intra-oral dental film packets. Upon exposure, the film produces a latent image that is only revealed after suitable processing. These film elements have historically been processed by treating the exposed film with at least a developing solution having a developing agent that acts to form an image in cooperation with other components in the film.
It is always desirable to limit the amount of solvent or processing chemicals used in the processing of silver-halide films. The traditional photographic processing scheme for black-and-white film involves development, fixing and washing, each step typically involving immersion in a tank holding the necessary chemical solution. By the use of photothermographic film, it is possible to eliminate processing solutions altogether, or alternatively, to minimize the amount of processing solutions and the complex chemicals contained therein. A photothermographic (PTG) film by definition is a film that requires energy, typically heat, to effectuate development. A dry photothermographic film requires only heat. A solution-minimized photothermographic film may require small amounts of aqueous alkaline solution to effectuate development, which amounts may only be that required to swell the film without excess solution. Development is the process whereby silver ion is reduced to metallic silver and in a color system, a dye is created in an image-wise fashion. In many photothermographic films, the silver is typically retained in the coating after the heat development.
In photothermographic films employing what is referred to as “dry physical development,” a photosensitive catalyst is generally a photographic-type photosensitive silver halide that is considered to be in catalytic proximity to a non-photosensitive source of reducible silver ions. Catalytic proximity requires intimate physical association of these two components either prior to or during the thermal image development process so that when silver atoms, (Ago)n, also known as silver specks, clusters, nuclei, or latent image, are generated by irradiation or light exposure of the photosensitive silver halide, those silver atoms are able to catalyze the reduction of the reducible silver ions within a catalytic sphere of influence around the silver atoms (Klosterboer, Neblette's Eighth Edition: Imaging Processes and Materials, Sturge, Walworth & Shepp (eds.), Van Nostrand-Reinhold, New York, Chapter 9, pages 279–291, 1989). It has long been understood that silver atoms act as a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed in catalytic proximity with the non-photosensitive source of reducible silver ions in a number of different ways (see, for example, Research Disclosure, June 1978, item 17029. Research Disclosure is a publication of Kenneth Mason Publications Ltd., Dudley House, 12 North Street, Emsworth, Hampshire PO10 7DQ England and also available from Emsworth Design Inc., 147 West 24th Street, New York, N.Y. 10011). Research Disclosure, September 1996, Number 389, Item 38957 is hereafter referred to as “Research Disclosure I”. 
The non-photo-sensitive source of reducible silver ions is typically a material that contains reducible silver ions and preferably a silver salt of an organic compound.
Photothermographic (PTG) media employing dry physical development are formulated with one or more light sensitive imaging layers on a light transmitting or reflecting support. Each imaging layer typically has at least one light-sensitive silver-halide emulsion, a reducible non-light-sensitive silver salt, a developer or developer precursor, and optionally a coupler to form dye. Other components may include accelerators, toners, binders, and antifoggants known in the trade as well as components used in conventional solution-processed silver-halide photographic media.
When exposed to light and then heated at temperatures ranging from 100 to 200° C. for 5 to 60 seconds, photothermographic media develop densities varying with exposure. The density versus log exposure curve (H&D curve) is commonly used in the trade to compare parameters such as speed and contrast. A typical procedure for generating the H&D curve entails making a contact exposure through a step tablet image. The steps modulate the intensity of the incident light, usually in 0.10 to 0.30 log exposure increments. Another method entails exposing pixel-wise using a laser, CRT or LED source in which the exposure intensity is modulated electronically. The H&D curve can also be determined using ionizing radiation such as x-radiation that is used in radiography. One method of determining the H&D curve with ionizing radiation is to vary the amount of ionizing radiation received by the photothermographic media by passing the ionizing radiation through an ionizing radiation absorbing step wedge before it impinges on the photothermographic element. Another method of determining the H&D curve with ionizing radiation is to perform successive exposures of the photothermographic element at different doses where the amount of ionizing radiation impinging on the photothermographic media for each exposure is determined by using an additional device (such as a dosimeter or radmeter) to measure the amount of ionizing radiation impinging on a surface for a given set of exposure conditions. In both methods the exposed media is processed after exposure then measured.
The measured reflection or transmission density of each step on the photographic media is then plotted against relative or absolute log exposure to produce what is known in the industry as the “H&D curve.” H&D curves typically have two plateaus corresponding to the maximum density (Dmax) and minimum density (Dmin) where the slope of the H&D curve approaches or equals zero; that is, a change in exposure produces little or no change in measured density. Gamma refers to the slope of the H&D curve usually at some fixed density position. Point gamma refers to the change in density between two adjacent exposure positions in a plot of the H&D values. The mid-scale density refers to the density midway between Dmax and Dmin plateaus, or (Dmax−Dmin)/2. The corresponding exposure is designated the midscale exposure.
As used herein with respect to the present invention, the term “negative-working” refers to a photographic silver-halide emulsion that develops more density with increasing exposure up to the Dmax limit when an imagewise-exposed gelatin coating of the emulsion is processed using a solution-development process and concomitant materials in accordance with the well-known and conventional D-76 standard. The corresponding H&D curve has a positive slope in the mid-scale density range when density is plotted against increasing relative log exposure. The unexposed areas develop to Dmin. The image produced in this way is referred to as a “negative image.” It is to be understood that the term “negative-working emulsion” as used herein is synonymous with “potentially negative-working emulsion” and refers to an inherent capability of the emulsion that may or may not be realized in practice. A “positive-working” photographic silver-halide emulsion, as used herein with respect to the present invention, responds to exposure by developing less density with increasing exposure down to the saturation limit (Dmin) when an imagewise-exposed gelatin coating of the emulsion is processed using a solution-development process and materials in accordance to the well-known D-76 standard. In this case, the H&D curve has a negative slope in the mid-scale density region when density is plotted against increasing relative log exposure. The unexposed areas develop to Dmax. The image produced in this way is referred to as a “positive image.”
Materials, including solution developers, qualifying for commercially acceptable use in a D-76 standard process include Kodak's trademarked products designed for such a process. See G. Haist, “Modern Photographic Processing, Vol 1”, John Wiley & Sons, Chapter 7, p 340 (1979) for the preparation of D-76 developer and other related developer formulas, the disclosure of which is hereby incorporated by reference. D-76 developer, therefore, includes any or all materials designated for and commercially used, with commercially satisfactory results in a D-76 process. Preferably, the D-76 developer is a Kodak product or one that is substantially equivalent in practice.
In a positive-working or negative-working emulsion, the developed density can comprise either silver, or if the imaging layer also contains a dye-forming coupler to react with oxidized developer, silver plus dye.
In the case of conventional solution-processed photographic media, as compared to dry or apparently dry thermally developed photothermographic media, positive images can be obtained from negative-working emulsions using combinations of multiple exposures and/or multiple development steps. See G. Haist, cited above, for details on black-and-white and color reversal-development processes, in which the following patents are cited: U.S. Pat. Nos. 2,005,837, 2,126,516, 2,184,013, 2,699,515, 3,361,564, 3,367,778, 3,455,235, 3,501,310, 3,519,428, 3,560,213, 3,579,345, 3,650,758, 3,655,390, BR 44248, BR 1151782, BR 1155404, BR 1186711, BR 1201792, CA 872180, and CA 872181.
For example, photobleach emulsions can be used in conventional solution-developed silver-halide photographic media to produce positive images. These emulsions are prepared with desensitizing dyes and chemical fogging agents. An exposure destroys preformed surface fog centers rendering the grains undevelopable. The unexposed grains develop to form a positive image. G. Haist reviews this topic in Modern Photographic Processing, Vol 2, Chapter 7, John Wiley & Sons, (copyright 1979).
GB 2018453A to Willis et al. teaches a photothermographic element comprising resorcinolic coupler, phenylenediamine developer, gelatin, silver bromoiodide emulsion (negative-working), various reducible organic silver salts (notably the silver salt of 3-amino-5-benzylthio-1,2,4-triazole (ABT)), and an antifoggant 3-methyl-5-mercapto-1,2,4-triazole (MMT). Slusarek et al., in U.S. Pat. No. 6,319,640 and U.S. Pat. No. 6,312,879, describes blocked phenylenediamine developers for photothermographic media coated from water and gelatin.
Negative-working photographic silver-halide emulsions are used in radiography for both industrial and medical applications. Negative-working photographic silver-halide emulsions can be used to image ionizing radiation directly or indirectly by the use of an intensifying element for ionizing radiation. An intensifying element is used for converting ionizing radiation to a lower-energy form suitable for exposing photographic or photothermographic elements. In radiography, intensifying elements are used in conjunction with photographic elements. Known intensifying elements include, for example, inorganic and organic phosphors as well as metal particles and metal foils. Intensifying elements in radiography can also be intensifying screens, imaging plates, radiographic screens, or phosphor screens. Most intensifying screens used in radiography contain luminescent materials called phosphors, scintillators, or luminophores. These materials, often in the form of particles, emit visible light upon irradiation with ionizing radiation. The light emitted by the phosphor leaves the intensifying element or screen and impinges on the negative-working photographic silver-halide emulsion to form the latent image that is subsequently developed imagewise.
Thus, radiographic films can be used in combination with some other material to convert the x radiation to another radiation form that can be more readily detected by silver halide in the films. Such radiation converting materials can be metal plates of metal oxides that convert x-radiation to electrons or can be inorganic phosphors that convert x-radiation to visible radiation. Such converting materials are usually provided in a separate element in what is known as “metal screens,” intensifying screens, or phosphor panels. If phosphors or metal oxides are included within the typical silver halide emulsion, image noise levels may increase. This is due to the fact that electrons or visible radiation from the converting materials may expose silver halide grains outside of the image area, giving rise to image noise. Thus metal or phosphor intensifying screens or panels may be preferred for use in combination with radiographic films in what are known as cassettes or radiographic imaging assemblies. However, the incorporation of phosphors in silver-halide emulsions are known. U.S. Pat. No. 6,440,944 teaches the use of negative imaging photothermographic elements with intensifying screens as well as the direct addition of x-ray sensitive phosphors to the photothermographic element to prepare a radiographic element suitable for imaging.
Negative-working photothermographic silver-halide emulsions are used in medical imaging as image-receiving elements in laser printing stations such as the KODAK DryView® laser printer. These laser printer stations are used to obtain hard copy images, of results from physical examinations of patients, taken using digital imaging modalities such as magnetic resonance imaging, ultrasound, positron emission tomography, computer aided tomography, and computed radiography. The negative-working photothermographic silver-halide emulsions used in these systems is not exposed to ionizing radiation. An example of a negative-working photothermographic silver-halide emulsion that can be used to image ionizing radiation is described in U.S. Pat. No. 6,440,649 by Simpson et al.
Historically, photographic films containing various silver halides have been used for various radiographic purposes. Such films have exhibited excellent sensitivity to x-radiation, high spatial resolution, low image noise, and archival storage properties. Desired sensitivity to imaging x-radiation has been achieved through amplification of a relatively small number of latent image centers without too much noise being added to the image. The term noise is understood in radiography to refer to the random variations in optical density throughout a radiographic image that impairs the user's ability to distinguish objects within the image. Radiographic noise is considered to have a number of components identified in the art as quantum mottle, film grain, and structure mottle as noted, for example, by Ter-Pogossian, THE PHYSICAL ASPECTS OF DIAGNOSTIC RADIOLOGY, Harper and Row, New York, Chapter 7, 1967.
Positive-working photographic silver-halide emulsions are not generally used for imaging ionizing radiation or in radiography. There are no known positive-working photothermographic silver-halide emulsions that are sensitive to ionizing radiation.
A significant problem with photothermographic elements has been the difficulty obtaining high photographic speeds. Silver-halide emulsions that are optimally sensitized for photographic speed in aqueous gelatin generally lose speed in contact with organic solvents and non-gelatin binders that are used in many non-aqueous photothermographic systems. Organic solvents may induce dye desorption, dye deaggregation, or some other chemical effect that degrades photographic efficiency. Methods of chemical and spectral sensitizations in organic solvents are less effective than in water for similar reasons.
Gelatin coatings, on the other hand, are more difficult to thermally develop due to the physical properties of the gelatin when it is heated. Lower developed density and photographic speed generally result from the higher glass transition temperature of gelatin and generally slower rates of diffusion of developer components in the strong hydrogen bonding polypeptide matrix. Gelatin coatings also require dispersing the incorporated water-insoluble developer components, which causes them to react generally more sluggishly under thermal processing conditions compared to organic solvent coatings in which developer components are dissolved in the coating solvent.
In addition, all of the prior art describes photothermographic systems that produce negative images that are nearly equal in speed to those obtained with solution development. In contrast, the present invention can produce direct-positive photographic speeds that are two to three stops greater than speeds obtained by solution or thermal development of same-size negative-working silver-halide emulsions.